r _vis viva_, which must now spend itself in
overcoming an _excess_ of resistance over and above the imposed
load, and the whole mass will not stop until the deflection (as
well as the resistance) has come to be equal to _twice_ that
corresponding to the static load imposed. Hence we say the
effect of a suddenly imposed load is to produce twice the
deflection and stress of the same load statically applied. It
must be evident, however, that this case has nothing in common
with either the ordinary 'static' tests of structural materials
in testing-machines, or with impact tests."[13]

[Footnote 13: Johnson, J.B.: The materials of construction, pp.
81-82.]

(3) ~Impact, shock,~ or ~blow.~[14] There are various common
uses of wood where the material is subjected to sudden shocks
and jars or impact. Such is the action on the felloes and spokes
of a wagon wheel passing over a rough road; on a hammer handle
when a blow is struck; on a maul when it strikes a wedge.

[Footnote 14: See Tiemann, Harry D.: The theory of impact and
its application to testing materials. Jour. Franklin Inst.,
Oct., Nov., 1909, pp. 235-259, 336-364.]

Resistance to impact is resistance to energy which is measured
by the product of the force into the space through which it
moves, or by the product of one-half the moving mass which
causes the shock into the square of its velocity. The work done
upon the piece at the instant the velocity is entirely removed
from the striking body is equal to the total energy of that
body. It is impossible, however, to get all of the energy of the
striking body stored in the specimen, though the greater the
mass and the shorter the space through which it moves, or, in
other words, the greater the proportion of weight and the
smaller the proportion of velocity making up the energy of the
striking body, the more energy the specimen will absorb. The
rest is lost in friction, vibrations, heat, and motion of the
anvil.

In impact the stresses produced become very complex and
difficult to measure, especially if the velocity is high, or the
mass of the beam itself is large compared to that of the weight.

The difficulties attending the measurement of the stresses
beyond the elastic limit are so great that commonly they are not
reckoned. Within the elastic limit the formulae for calculating
the stresses are based on the assumption that the deflection is
proportional to the stress in this case as in static tests.

A comm